The present invention relates to a method of producing calcium carbonate from lime which is used to produce a solution of calcium ions to which carbon dioxide is added to precipitate calcium carbonate.
Calcium carbonate has a wide variety of uses. For example, it is used extensively as a functional filler in materials such as paints, paper, coatings, plastics, sealants and inks. Other applications of calcium carbonate are in the food, cosmetics and pharmaceutical industries.
Calcium carbonate is a naturally occurring mineral which after grinding is used for a number of applications. That said, the morphology, the particle size, and particle size distribution of the ground product as well as its purity are not adequate for certain applications nor optimum for others.
Calcium carbonate may also be obtained by a “chemical route” in which carbon dioxide is added to a solution of calcium ions, resulting in precipitation of calcium carbonate. The starting material for such a process is typically lime (CaO) or lime hydroxide (Ca(OH)2). The “classic” process uses CaO as the starting material. Initially this lime is slaked with water to produce an aqueous suspension of lime hydroxide (“slaked lime”):CaO+H20=Ca(OH)2 to which carbon dioxide is added to produce calcium carbonate in accordance with the following equation:Ca(OH)2+C02=CaC03+H20
There is however a disadvantage associated with the “classic” process in that the Ca(OH)2 has very limited solubility in water so the overall process is relatively slow.
In a development of the above process a solution of calcium ions is prepared by dissolving the lime or lime hydroxide in an aqueous solution incorporating a polyhydroxy compound which serves to facilitate dissolution of calcium ions. As a result, the carbonation process is faster. Various polyhydroxy compounds may be used for this purpose. For example, WO-A-0034182 (Kemgas Ltd) discloses the use of polyhydroxy compounds of formula HOCH2(CHOH)nCH2OH in which n is 1 to 6 with the preferred example being sorbitol. Other polyhydroxy compounds that may be used to facilitate the solution of the lime or lime hydroxide (to produce a calcium ion solution to which carbon dioxide is added to precipitate carbon dioxide) include sucrose.
The production of calcium carbonate using the process described in WO-A-0034182 has a particular advantage in that the lime or lime hydroxide may be a waste, by-product from another chemical process, so that its conversion to calcium carbonate allows a useful material to be generated from what would otherwise be waste. Thus, for example, the waste lime may be carbide lime which is a by-product in the production of acetylene by the reaction of calcium carbide and water according to the equationCaC2+2H20Ca(OH)2+C2H2 
Carbide lime is also known as carbide sludge, generator slurry, lime sludge, lime hydrate, and hydrated carbide lime. It is a grey-black substance typically consisting of around 90% by weight of calcium hydroxide (based upon the solids content of the carbide lime), the remainder being impurities which depend upon the method used to manufacture the acetylene and also upon the source of the materials used to manufacture the calcium carbide (normally made by roasting calcium oxide and coal). The main impurities are the oxides of silicon, iron, aluminum, magnesium and manganese combined with carbon, ferrosilicon and calcium sulphate. Additionally if the carbide lime is stored outside, calcium carbonate, formed by the reaction of calcium hydroxide with carbon dioxide, may be present as an impurity. The conversion of carbide lime to calcium carbonate is described in WO-A-0034182 (Kemgas Ltd).
The carbon dioxide required for the precipitation of calcium carbonate from the solution of calcium ions may also be a waste material whose discharge needs to be controlled for environmental reasons (it is a well-known “greenhouse gas”) so it may be effectively captured by conversion to calcium carbonate.
The production of calcium carbonate from waste lime or lime hydroxide and carbon dioxide therefore represents an environmentally attractive process.
There are several disadvantages associated with the production of calcium carbonate by the carbonation of a solution of calcium ions obtained from lime or its hydroxide using the “classic” process. These are described below with specific reference to lime although it will appreciated that the same disadvantages apply to the use of lime hydroxide as starting material.
Firstly, the calcium carbonate produced from lime may incorporate unacceptably high levels of impurities derived from the latter. There are at least two sources of such impurities in the lime. One source is “naturally occurring” in that the lime will itself (usually) be derived from a mineral source of calcium carbonate (e.g. limestone, dolomite etc) and such minerals will include impurities which remain in the lime. A further (possible) source of impurities in the lime applies particularly in the case of lime hydroxide and arises from a chemical reaction by which the lime hydroxide has been produced (e.g carbide lime may incorporate impurities from carbon used in the production of calcium carbide). Whatever the source, examples of impurities present in a calcium carbonate product obtained by carbonation of a solution of calcium ions derived from lime or lime hydroxide may include aluminum, arsenic, lead, iron, mercury, chromium, nickel, copper and/or zinc. Some of these impurities render calcium carbonate unsuitable for certain applications (e.g. food and pharmaceutical uses) due to toxicity issues. Other impurities (e.g. iron) whilst not necessarily being toxic may affect properties such as the “whiteness” of the calcium carbonate so that it may not be suitable, for example, for use as a coating in high quality papers.
Secondly calcium carbonate produced by the classic process involving addition of carbon dioxide to a suspension of slaked lime may have irregular particle sizes and shapes and the particles may form agglomerates rather than remaining as discrete particles. Irregular particle sizes cause problems in applications such as polymers, sealants, decorative paints, industrial coatings, inks and paper coating.
Thirdly for certain applications the conventional process, due to the irregular particle sizes, requires subsequent milling of the product. Milling is energy intensive and always creates a certain amount of fine particles which can be detrimental and are difficult to remove.
It is therefore an object of the present invention to obviate or mitigate the above mentioned disadvantages.
According to the present invention there is provided a method of producing calcium carbonate from lime comprising the steps of:
(i) providing an aqueous solution comprising 10% to 35% by weight of dissolved polyhydroxy compound and 1% to 5% by weight of dissolved calcium hydroxide (expressed as Ca(OH)2) and having a pH of at least 11.5;
(ii) treating the solution prepared in step (i) to remove solids including suspended solids;
(iii) dispersing carbon dioxide through the solution so as to form calcium carbonate with a consequential reduction in the pH of the reaction mixture;
(iv) during a time period beginning at the start of a sudden, short rise in pH and ended during a subsequent fall in pH but before the pH reaches 9.5 terminating the dispersion of carbon dioxide and adding an alkaline reagent to maintain a pH for the product mixture of at least 9.5; and(v) recovering precipitated calcium carbonate.
We have established that the carbonation reaction involving a dispersion of carbon dioxide through an aqueous solution of lime dissolved with the aid of a polyhydroxy compound proceeds in a number of phases.
In the first phase (“phase 1”) viscosity remains stable, carbon dioxide can be absorbed at a relatively high rate, and there is a gradual fall in pH. After a particular time depending on reaction conditions, there is a transition from the from the first phase to a second phase (“phase 2”) in which the maximum rate at which carbon dioxide can be absorbed by the reaction mixture is lower than in phase 1 of the process. The transition from phase 1 to phase 2 may be detected by an increase in the amount of carbon dioxide passing out of the reactor in which the carbonation reaction is being effected (assuming that the carbon dioxide is supplied to the reactor at the same rate as in phase 1). Depending on reaction conditions, there may be a viscosity increase in going from phase 1 to phase 2 and a gel may be visible for the duration of the latter. The pH continues to fall gradually during phase 2, although generally will remain above 10.
Surprisingly, there is a sudden, short rise in pH followed by a decline which if left uncontrolled results in the pH of the product mixture continuing to fall. We identify the beginning of this short, sharp rise in pH as the commencement of a third phase (“phase 3”) for the reaction. During phase 3 calcium carbonate particles precipitate out and there is generally an increase in opacity caused by precipitated calcium carbonate particles. The increase in opacity is notable by a visible brightening of the reaction mixture with break-up of any gel.
Based on our studies of the reaction (as detailed above) we have established that calcium carbonate of high purity and small, uniform particle size may be produced by control of the reaction in phase 3 by:
(a) terminating dispersion of carbon dioxide into the reaction mixture after the beginning of phase 3 (manifested by the short, sharp rise in pH) but before pH drops below 9.5; and
(b) ensuring that the pH of the product mixture is maintained at a value of at least 9.5 by the addition of an alkaline reagent.
By adopting these two control features we have established that calcium carbonate of small uniform particle size and high purity is produced.
In a preferred embodiment of the method of the invention, the rate at which carbon dioxide is dispersed in the reaction mixture during phase 1 of the method is the maximum rate at which the carbon dioxide can be absorbed so that little or no carbon dioxide is evolved from the reaction mixture. This minimises the time required for completion of phase 1 which improves the productivity of the process.
Similarly phase 2 is also preferably effected using a rate of carbon dioxide dispersion which is the maximum that can be absorbed by the reaction mixture. It will however generally be found that the rate (for phase 2) is lower than for phase 1.
Preferably addition of carbon dioxide is terminated after the beginning of the sudden short pH rise and prior to addition of the alkaline reagent. This ensures that the carbon dioxide does not contribute to a pH reduction after addition of the alkaline reagent.
Calcium carbonate produced by the method of the invention has a very narrow particle size distribution. This can be expressed by the ratio d90/d0 where d90 is the size below which 90% of particles fall and di0 is the size below which 10% of particles fall. This ratio is typically below 4.0. Typically the method of the invention allows calcium carbonate to be produced with mean particle sizes from 0.3 to 3.0 microns. A typical example of particles produced in accordance with the method of the invention have a size of about 0.9 microns with a particle size distribution giving a d90 of about 1.3 microns and a d0 of about 0.5 microns so that the ratio d90/d10 is 2.6. Particle sizes have been measured with a Beckman Coulter laser diffraction particle size analyzer.
When seen under high magnification, particles of calcium carbonate obtained in accordance with the invention have the appearance of “rice grains” in that they are elongate with rounded ends. The aspect ratio (length divided by diameter) is between 1.5 and 5.0, and is typically 3.0.
Calcium carbonate produced in accordance with preferred embodiments of the invention may have impurity levels as set out in the following table:
CompoundConcentration ± 10%Al Aluminium<10ppmAs Arsenic<0.2ppmPb Lead<0.2 ppm (200 ppb)Fe Iron<20ppmHg Mercury<0.02ppmCr Chromium<1.6ppmNi Nickel<3.7ppmCu Copper<0.3ppmZn Zinc<1ppm
Calcium carbonate produced in accordance with the invention has a number of advantages. For example, it has good values for both brightness, light scattering and gloss as well as low abrasivity which makes it particularly suitable for use in the paper coating and polymer industry, particularly in view of the very regular “rice grain” crystal morphology which leads to superior Theological effects. Additionally the purity of the product and the absence therefrom of large particles provide for very low abrasivity. Calcium carbonate produced in accordance with the invention also has a wide variety of applications beyond the paper coating and polymer industries. For example, its low level of impurities (particularly low level of lead) makes it suitable for food, pharmaceutical industries.
The brightness of the calcium carbonate is a good indication of its purity. Calcium carbonate produced in accordance with the invention will generally have a brightness R457 equal or superior to 96.0.
Sources of lime or lime hydroxides that may be used for producing calcium carbonate in accordance with the method of the invention include, for example, burnt lime produced by the calcination of limestone, carbide lime and other waste limes or lime hydroxides. Additional sources of calcium oxide or hydroxide may include Paper Sludge Ash, the product of incinerating paper sludge, in particular the sludge waste stream from the deinking of pulp recovered from recycled paper. The incineration of the paper sludge produces calcium oxide. The calcium oxide component provides the source of calcium ions dissolved in the polyhydroxide containing solution
Step (i) of the method of the invention involves the production of a solution of calcium ions (derived from the starting lime or lime hydroxide) in a polyhydroxy compound which promotes the dissolution of the calcium. The final solution produced comprises 10% to 35% by weight of the dissolved polyhydroxy compound and 1% to 5% by weight of dissolved lime hydroxide (expressed as Ca (OH)2). The solution has a pH of at least 11.5, but usually at least 12. If the starting material is lime (CaO) then it is generally preferred initially to produce a slurry of lime hydroxide (“slaked lime”) and admix this slurry with a solution of the polyhydroxy compound so as to produce a final solution comprising 10% to 35% by weight of dissolved polyhydroxy compound and 1% to 5% by weight of dissolved lime hydroxide (expressed as Ca(OH)2). If the starting material is a lime hydroxide then it may be admixed directly with the solution of the polyhydroxy compound.
As a general rule, the greater the amount of the dissolved polyhydroxy compound the greater is the amount of calcium ions that may be dissolved therein. Thus, for example, if the solution contains about 12% of the polyhydroxy compound then the amount of calcium hydroxide (expressed as Ca(OH)2) that may be dissolved therein will be a maximum of about 2%. As a further example, a solution containing about 25% by weight of the polyhydroxy compound can contain a maximum of about 4% of dissolved calcium hydroxide.
Whilst the method of the invention is effective using amounts of 10% to 35% by weight of dissolved polyhydroxy compound and 1% to 5% by weight of dissolved calcium hydroxide, we particularly prefer that the amount of polyhydroxy compound is in the range 20% to 30% and the amount of dissolved calcium hydroxide is 2 to 4.5%. More particularly, we prefer that the amount of the polyhydroxy compound is in the range 23% to 27% and the amount of dissolved calcium hydroxide is in the range 3 to 4.0%. Particularly good results are obtained using about 25% by weight of dissolved polyhydroxy compound and about 3.4% to 3.9% by weight of the dissolved calcium hydroxide.
Examples of polyhydroxy compounds which may be employed for the method of the invention are as disclosed in WO-A-0034182 (Kemgas Ltd) and include compounds of the formula:HOCH2(CHOH)nCH2OHwhere n is 1 to 6. Thus for example the polyhydroxy compound may be glycerol (n=1). It is however more preferred that n is 2 to 6 and is particularly preferred that the polyhydroxy compound is a sugar alcohol (a “hydrogenated monosaccharide”). Examples of sugar alcohols include sorbitol, mannitol, xylitol, threitol and erythritol.
Also useful as polyhydroxy compounds that may be employed in the invention are those having a straight chain of n carbon atoms where n is 4 to 8 and (n−1) of the carbon atoms have a hydroxyl group bonded thereto. The other carbon atom (i.e. the one without the hydroxyl group) may have a saccharide residue bonded thereto. Such compounds are hydrogenated disaccharide alcohols and examples include maltitol and lactitol.
Particularly preferred for use in the invention are the hydrogenated monosaccharide (e.g. sorbitol) and disaccharide alcohols because of their thermal stability which can be important for subsequent processing of the calcium ion solution (see below).
Mixtures of the above described polyhydric alcohols may be used. Thus it is possible to use industrial sorbitol which, of the solids present, comprise about 80% sorbitol together with other polyhydroxy compounds such as mannitol and disaccharide alcohols. Examples of industrial sorbitol include Sorbidex NC 16205 from Cerestar and Meritol 160 from Syral.
Additionally however the polyhydroxy compound may be a saccharide (e.g. a mono- or di-saccharide).
The solution prepared for step (i) of the process is then treated in step (ii) to remove insoluble material including suspended solids which will contain metal impurities, this being one step which results in the purity of the calcium carbonate product obtained by the method of the invention. It is particularly preferred that suspended solids are removed by a flocculation step. The flocculating agent used may, for example, be a cationic polymer (such as Nalco 9908) which is added to the solution with mixing. Floes and solids may be separated from the solution by conventional techniques. Thus, for example, the solution may be passed to a “settler” which allows the floes to be collected at, and discarded from, the bottom thereof. The solution may then be filtered through a sand column, or any other appropriate device, to remove remaining solid material.
The solution obtained from step (ii) is then subjected to a carbonation reaction (step iii) in which carbon dioxide either pure or diluted (if for instance a flue gas is used) is bubbled through the solution.
It is preferred that the reaction is effected in a batch reactor with a high shear gas dispersion agitator. However it is also possible to perform the reaction continuously either in a series of reactors with high shear agitators or in-line, adding the gas via ejectors in one or more steps. The amount of carbon dioxide added should be at least the stoichiometric amount required for conversion of all calcium ions in the solution to calcium carbonate.
The solution to be carbonated will typically be at a controlled temperature at the start of the carbonation reaction. Starting temperature will preferably be in a range of 10 to 40° C., and ideally in a range of 25 to 32° C.
During the course of a typical reaction, the pH (which is initially at least 11.5, more usually at least 12) progressively decreases. At a certain moment in the reaction there is a marked increase in the viscosity of the solution. We call this phase 2. Depending on the particular concentrations of polyhydroxy compound and calcium hydroxide in the starting solution this increase in viscosity may be caused by gel formation. Our studies have established that the progressive decrease in pH of the reaction mixture abruptly changes usually at a value of about 10.2-10.8 to a sudden sharp rise of typically 0.5 to 1 pH unit before continuing to decrease again.
The start of the short, sharp rise in pH denotes the end of phase 2 and during the period of the rise the calcium carbonate particles precipitate rapidly. As stated above, the amount of carbon dioxide to be added during the reaction should be at least the stoichiometric amount required for conversion of all calcium ions in the solution to carbon carbonate. Under the conditions described herein to make a 0.8 micron particle the quantity of carbon dioxide injected during phase 1 is between 70 and 85% of the total, with the remainder being injected in phase 2. Flow rates are generally as high as process conditions will allow. Those in phase 1 are generally much higher than in phase 2. Typically a reaction takes between 15 and 30 minutes.
An important feature of the invention is that once the pH of the product mixture begins to decrease after its short sharp rise it is not allowed to fall below 9.5, preferably not less than 10 and is ideally maintained at a value of at least 10.5.
We have established that this tight control of the pH of the product mixture (rather than simply allowing the pH to fall to lower values) is important in ensuring production of calcium carbonate of small uniform particle size and purity of the final calcium carbonate product. More particularly, we have found that some of the metal impurities present in the lime go into solution in step (i) of the method (e.g. by chelation with the polyhydroxy compound) and are therefore not removed in solids separation step (ii). By ensuring that the pH of the product mixture does not fall below 9.5, most of these metal impurities remain in solution and therefore do not contaminate the final precipitated calcium carbonate.
The arrest of the pH fall may be achieved by addition, to the product mixture, of an alkaline reagent. Most preferably the alkaline reagent is added to the product mixture as soon as practicably possible once the pH begins to fall after its short rise and in any event in time to ensure that the pH does not fall below 9.5. The alkaline agent should be one which does not lead to the introduction of impurities into the precipitated calcium carbonate product. For this reason, it is highly preferred to use as the alkaline agent a solution such as obtained from step (ii) as outlined above since the dissolved metal impurities contained therein do not precipitate to any substantial extent under the pH conditions prevailing in the product mixture. Typically the amount of this solution used will be 3 to 8% by volume of the product mixture so as to achieve the required arrest of the pH fall.
The solid calcium carbonate may be separated from the product mixture by any conventional separation technique. Thus, for example, a filter press may be used.
The liquor separated from the product mixture contains polyhydroxy compound which, ideally, is recycled for the purposes of producing a solution as required by step (i) of the method. This assists with the overall economics of the process. For this purpose, the separated liquor is purified and concentrated before being returned to step (i). Purification serves to remove impurities which might otherwise pollute calcium carbonate produced from the recycled solution of polyhydroxy compound. Purification is most conveniently effected by reducing the pH of the liquor to a value of 7 to 8 by addition of carbon dioxide. Subsequently the solution is subjected to evaporation to increase its concentration to a value appropriate for use in step (i) of the method. Evaporation should be effected under conditions that do not cause any significant decomposition of the polyhydroxy compound. Vacuum evaporation is preferred. After the evaporation step, the remaining solid contaminants are removed from the solution, for example by a second flocculation and a filtration or sedimentation step, as described above for step (ii), but not necessarily with both.